Tumor cells exhibit an enhanced ability to survive and proliferate in highly unfavorable environments. They have been shown to down-regulate many of the cellular pathways that prevent normal (i.e., non-cancerous) cells from dividing in a hostile environment, and they also inactivate apoptotic pathways that bring about cell death in many normal tissues under adverse conditions. Tumor cells are also believed to up-regulate pathways required to maintain active proliferation. For example, many tumor cells activate the cellular stress-response pathway that allows tumor cells to synthesize and maintain the protein machinery they need to continue proliferating. The activated stress response in tumors includes up-regulation of heat-shock proteins (Hsps), which are ATPase-directed molecular chaperones. In particular, Hsp90 is upregulated in many cancerous tissues. Hsp90 controls the balance between folding/maturation and proteasomal destruction of a restricted number of client proteins, some of which are involved in signal transduction and cell proliferation.